1. Field of the Invention
The present invention relates to an operational amplifier for amplifying a signal. More particularly, it relates to an operational amplifier used for a deflector in a scanning electron microscope.
2. Description of the Related Art
In an integrated operational amplifier, because of the limits to its manufacturing process and package technology, it is generally difficult to deal with a voltage exceeding ±20 V or a current exceeding a few 100 mA. Meanwhile, in an operational amplifier using discrete transistors, it has been difficult to obtain excellent offset-voltage drift characteristics, high voltage gain, and high slew rate. Also, trying to obtain the high voltage gain has made it difficult to secure stability at the time when a negative feedback is applied as the operational amplifier.
In a conventional example of power amplifiers illustrated in FIG. 2, an IC-implemented transistor pair whose withstand voltage is low is used as a first-stage differential pair. Moreover, voltage amplification is performed by a base-grounded circuit which is cascode-connected to this differential pair.
A differential amplifier, however, is used at a second stage. As a result, there has existed a drawback that recovery from supersaturation is slow. In addition, there has existed a drawback that a sufficient open voltage gain cannot be obtained due to a load effect on the first stage by the second-stage differential amplifier.
There has also been a necessity for implementing a unit for facilitating phase compensation at the expense of direct-current characteristics. Here, this unit facilitates the phase compensation by lowering the voltage gain through insertion of a resistor into emitters of the first-stage differential pair.
Also, the value of a capacitor for the phase compensation is a small value which can be influenced by even a parasite capacity of each portion in the individual components. Accordingly, there has also been a necessity for paying the most careful attention to implementation of this value. The above-described problems, however, cannot be solved by operational amplifiers described in JP-A-8-125474; Tutomu Okayama, “Introduction to Practical Analog Circuit Design and Analysis”, the Nikkan Kougyo Shinnbunn, p 200, FIG. 6. 24; Bob Widlar, “IC OP AMP BEATS FETS ON INPUT CURRENT”, EEE, December 1969; and “History of OP Amplifier and Basic Knowledge of Circuit Technologies”, CQ Publishing Corp., p 57, FIG. 3 to FIG. 5.